In order to promote adhesion between rubber and ferrous metals, it is known to employ a variety of metallic salts as coatings to the metal or as ingredients in the rubber composition. Exemplary of the former coating technique is U.S. Pat. No. 1,919,718 which is directed toward improving the adhesion between rubber and metal by coating the metal with a rubber cement which comprises a metal salt of a low molecular weight fatty acid.
Typical of the latter technique of the incorporation of the salt into the composition is U.S. Pat. No. 2,912,355, which is directed toward improving the adhesion between rubber and metal by the incorporation into a rubber composition of a calcined and thus partially to fully oxidized metal salt of an aliphatic fatty acid compound. The metal can be cobalt, copper, iron, lead, mercury, nickel or silver.
It is also known to add various resins as tackifiers and/or adhesion promoters and, in other instances, to employ both a metal salt and a resin. For example, the use of a rosin-derived resin in combination with a nickel or cobalt inorganic salt in a rubber composition to improve adhesion is disclosed in U.S. Pat. No. 4,258,770 owned by the common Assignee. The use of a thermoplastic resin, derived from crude wood rosin containing carboxylic acid groups or carbon carboxylic acid ester groups, in combination with an organic salt of nickel to improve adhesion is disclosed in U.S. Pat. No. 4,435,477 also owned by the common Assignee. However, none of these resins are thermosetting but rather they are thermoplastic in nature. Moreover these resins are all naturally occurring resins rather than synthetic.
With respect to the known uses of thermosetting two-step phenolic resins, typical is U.S. Pat. No. 2,822,026, owned by the common Assignee, which is directed toward the use of a thermosetting two-step phenolic resin and a rubber composition to prevent or retard rubber flow during vulcanization thereby providing a sufficient quantity of rubber in the bead area of the tire. However, there is no disclosure of the use of a nickel salt in combination with the resin or that the resin alone or in combination improves adhesion properties.
Finally, U.S. Pat. No. 4,383,005 is directed toward a rubber composition having improved metal adhesion properties wherein the composition contains a reinforcing filler containing active silicic acid and at least one resin derived from polyhydric phenols and aldehydes. However there is no disclosure of the use of metal salts or that improved adhesion results in stocks that do not contain silica.
While others have sought to enhance adhesion between rubber compositions and metals by employing various combinations of nickel salts and other metal salts with various resins, the art of has been presented herein has not disclosed the exclusive use of an organic salt of nickel or cobalt with a thermosetting two-step phenolic resin to increase adhesion properties beween rubber and metal reinforcements.